How to enter deep software standby mode?

Question:

I cannot enter deep software standby mode.

Answer:

The deep standby cancel flags in the deep standby interrupt flag register (DPSIFR) may be set to 1 when a cancel request is generated, even when not in deep software standby mode.
To enter deep software standby mode, set each deep standby cancel flag to 0 (no cancel request is generated).

For details, see Deep Standby Interrupt Flag Register (DPSIFR) and Flowchart to Use Deep Software Standby Mode in the Low Power Consumption chapter of the user's manual: hardware.